MECHANICAL SEPARATION 285 that the diminished proportion of water to be removed begins to play an important part in the computations. When this point is reached the capacity begins to rise and the capacity figure at the turn represents the safe working capacity of the tank, one which will take care of all reasonable variations in proportions of water and solids entering a tank. Let it be supposed that a test sample has been prepared showing a proportion of water to solids of 14.04 to 1 and that after 17 hr. the pulp has settled to 1.13 parts of water to 1 of solids and only to 1.12 to 1 after 24 hours, then evidently the economic point of thickening is 1.13 to 1. Settlement tests show an average rate of 1.78 in. per hour, hence there can be decanted 1.78 cu. ft. times 62.3 or 111 Ib. of water per hour per square foot of tank surface. Since the feed contains 14.04 water to 1 of solids and the discharge 1.13 water to 1 of solids the overflow must contain the difference between the two ratios or 12.91 parts of water in the overflow to 1 part solids settled and is 111 Ib. per hour. The solids consequently represent 8.59 Ib. solids settled per hour or 206 Ib. of solids in 24 hr. For a ton there would then be required 9.7 sq. ft. for 24 hr. which is the capacity of the tank with the dilution given. The computations may be expressed in the following formula Square feet of tank surface required per ton dry per 24 hr. = jgv-62 3X24~ where R = Rate of settlement in feet per hour, F — Parts of water to 1 part solids in feed, D = Parts of water to 1 part solids in discharge. The series of tests which follow the first one will give the following factors and capacities. Square feet tank R F F - D surface required per dry ton per 24 hr. 1.250 11.18 10.05 10.70 0.666 8.32 7.19 14.40 0.464 6.89 5.75 16.60 0.345 5.47 4.34 16.85 0.2331 4.03 2.90 16.65 0.150 2.607 1.477 13.10 The series of tests show that 16.85 sq. ft. of surface will be required for each ton settled. With 8 parts of liquid to one of solids the settling area required by Dorr thickeners ranges from 5 to 25 sq. ft. per ton of solids. Electrolytes.—By the use of electrolytes in the solution the settlement area may often be reduced and they can be used if they give results and have no harmful effect. Different acids and salts have varying flocculating effect. Alkalies in small percentages have a deflocculating effect. Less than 1.00 per cent of caustic potash has a decided deflocculating effect. Above this proportion the effect is flocculating. Very dilute salt solutions give a good flocculating effect but greater proportions do not cause any further gain in settling effect. Common salt solutions have not a very strong flocculating effect but the maximum effect is obtained when the concentration is about 0.10 per cent.